A beach is a sloping stretch of sand, shingle, gravel, and other material along the shore of a lake, river, or sea. Caused by erosion and deposition of sediment by waves, a beach usually extends from the furthest point reached by the waves out to a water depth of around 10 meters. Waves breaking on the beach move the sediment back and forth so the heavier pebbles remain on the upper beach and the sand moves downward toward low water.